Other Accrued Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Components of other accrued liabilities
Accrued systems rental and maintenance costs	$ 1,320	$ 1,488
Accrued site rental fees	938	999
Accrued property and sales taxes payable	9,435	10,253
Accrued self-insurance expense	1,734	1,635
Deferred gain on sale of equipment	312	319
Other accrued expenses	9,056	7,308
Total	$ 22,795	$ 22,002